In the past a tremendous amount of work has been devoted to the manufacture of sulphur-containing fertilizers. The growing worldwide demand for sulphur-containing fertilizers stems from the discovery that low crop yields in certain cases may be related to deficiencies in sulphur in the soil. An example of a species with high sulphur requirements is Canola. Canola is an important cash crop in Alberta, Canada, and has high sulphur requirements at any growth stage. A shortage of sulphur can cause serious reductions in crop yield.
Manufacturing processes for sulphur-containing fertilizers of the ammonium phosphate type often involve the use or incorporation of sulphates, see e.g. U.S. Pat. Nos. 4,377,406, or 4,762,546. A disadvantage of sulphates is that they are very mobile in the soil and leachable. Elemental sulphur is not leached out of the soil, as are sulphates. It is therefore more advantageous to have the sulphur present as elemental sulphur. Furthermore, elemental sulphur offers some additional benefits in fertilizers: elemental sulphur acts as a fungicide against certain micro organisms, as a pesticide against certain soil and plant pests, it assists the decomposition of plant residues and it improves phosphorus and nitrogen utilization and reduces the pH of alkaline and calcareous soils.
Thus, it is advantageous to incorporate sulphur as elemental sulphur in the sulphur-containing fertilizers.
Processes for the manufacture of sulphur-containing fertilizers, wherein elemental sulphur is used, are known in the art. Most of the methods involve the incorporation of molten sulphur into the fertilizer.
In U.S. Pat. No. 5,653,782, a process for the manufacture of sulphur-containing fertilizers has been described, wherein a substrate containing fertilizer particles is heated to a temperature above the melting point of sulphur and admixed with sulphur. According to U.S. Pat. No. 5,653,782, the sulphur is melted by the heat provided by the preheated fertilizer particles, thereby producing a homogeneous coating on the fertilizer particles.
U.S. Pat. No. 3,333,939, describes the coating of ammonium phosphate granules with molten sulphur. The granules are coated in a separate coating unit into which the sulphur is fed, by contacting the granules with molten sulphur or with a solution of ammonium polysulphide. Subsequently, the coated granules are dried. Alternatively, U.S. Pat. No. 3,333,939 teaches a process for preparing sulphur-containing fertilizer particles in which the sulphur is interspersed throughout the particles. In this process ammonia and phosphoric acid are allowed to react to form ammonium phosphate. The ammonium phosphate formed is fed into a granulator in which it is mixed with urea and dry sulphur. The granules obtained are dried in a dryer. The disadvantage of the first process of U.S. Pat. No. 3,333,939 is that the coating prevents a uniform distribution of ammonium sulphate and sulphur into the soil. The second process has the disadvantage that it requires solid sulphur handling. The handling and grinding of solid sulphur is highly hazardous due to the dust and risks of explosions. As mentioned in a review by H. P. Rothbaum et al (New Zealand Journal of Science, 1980, vol. 23, 377), explosion hazards are always due to sulphur dust which is inflammable. Therefore, a more complex process design is necessary to ensure the safety of the process.
U.S. Pat. No. 5,571,303 discloses a process for the manufacture of fertilizers in which first ammonia, water and phosphoric acid are reacted to form ammonium phosphate. Subsequently, the ammonium phosphate/water mixture is mixed with molten sulphur. The mixture thus obtained is kept at temperatures of 120-150° C. until granulation. A disadvantage of this process is that due to the preforming of ammonium phosphate either much water is needed to keep the salt dissolved or, when relatively small amounts of water are used, solid ammonium phosphate is formed. The homogeneous distribution of sulphur throughout the granule may be hampered by the existence of solid ammonium phosphate.
Since problems with the manufacture of fertilizers of the sulphur-containing ammonium-phosphate type, continue to be in existence, there is a need for a manufacturing process for such fertilizers which would diminish or even prevent the problems experienced in the art.
It has now been found that a process for manufacturing sulphur-containing fertilizers, wherein sulphur is introduced as a liquid phase comprising elemental sulphur, offers advantages over the manufacturing processes known in the art, with regard to safety aspects as well as with respect to process control.
The process according to the invention enables the manufacture of fertilizers with an even distribution of sulphur throughout the fertilizer product, thereby enhancing conversion in the soil to plant usable form, namely sulphates. The fertilizer is thus enabled to deliver sulphates to the intended crop in a more reliable and consistent manner.